ATTINY87 Datasheet, PDF (8/282 Pages) ATMEL Corporation – 8-bit Microcontroller with 8K/16K Bytes In-System Programmable Flash and LIN Controller

Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data
Space addressing – enabling efficient address calculations. One of the these address pointers
can also be used as an address pointer for look up tables in Flash Program memory. These
added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.
The ALU supports arithmetic and logic operations between registers or between a constant
and a register. Single register operations can also be executed in the ALU. After an arithmetic
operation, the Status Register is updated to reflect information about the result of the
operation.
Program flow is provided by conditional and unconditional jump and call instructions, able to
directly address the whole address space. Most AVR instructions have a single 16-bit word
format. Every Program memory address contains a 16- or 32-bit instruction.
During interrupts and subroutine calls, the return address Program Counter (PC) is stored on
the Stack. The Stack is effectively allocated in the general data SRAM, and consequently the
Stack size is only limited by the total SRAM size and the usage of the SRAM. All user pro-
grams must initialize the SP in the Reset routine (before subroutines or interrupts are
executed). The Stack Pointer (SP) is read/write accessible in the I/O space. The data SRAM
can easily be accessed through the five different addressing modes supported in the AVR
architecture.
The memory spaces in the AVR architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional Global
Interrupt Enable bit in the Status Register. All interrupts have a separate Interrupt Vector in the
Interrupt Vector table. The interrupts have priority in accordance with their Interrupt Vector
position. The lower the Interrupt Vector address, the higher the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as Control Regis-
ters, SPI, and other I/O functions. The I/O memory can be accessed directly, or as the Data
Space locations following those of the Register File, 0x20 - 0x5F.
2.2 ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose
working registers. Within a single clock cycle, arithmetic operations between general purpose
registers or between a register and an immediate are executed. The ALU operations are
divided into three main categories – arithmetic, logical, and bit-functions. Some implementa-
tions of the architecture also provide a powerful multiplier supporting both signed/unsigned
multiplication and fractional format. See the “Instruction Set” section for a detailed description.
2.3 Status Register
The Status Register contains information about the result of the most recently executed arith-
metic instruction. This information can be used for altering program flow in order to perform
conditional operations. Note that the Status Register is updated after all ALU operations, as
specified in the Instruction Set Reference. This will in many cases remove the need for using
the dedicated compare instructions, resulting in faster and more compact code.
The Status Register is not automatically stored when entering an interrupt routine and
restored when returning from an interrupt. This must be handled by software.
8 ATtiny87/ATtiny167
7728D–AVR–07/09